1. Field of the Invention
The present invention relates to a method for receiving an OFDM signal, and a receiver.
2. Description of the Related Art
A transmitter of a radio communication system using an orthogonal frequency division multiplexing (OFDM) technique (OFDM communication system) allocates modulated symbols obtained by applying an orthogonal modulation to an information signal to be transmitted to each subcarrier. The transmitter generates an OFDM signal by applying inverse fast Fourier transform (IFFT) to each subcarrier with modulated symbols allocated thereto, and radio-packetizes the OFDM signal to transmit it. On the other hand, a receiver of the OFDM communication system receives the transmitted OFDM signal to demodulate it.
In the receiver, it is necessary to compensate for the carrier offset and clock offset caused by frequency deviations between a crystal oscillator in the transmitter and that of the receiver. The carrier offset is generated in down converting of a received signal in a baseband signal. The clock offset is generated in analog-digital converting the received signal, and results in occurrence of a conversion error in a digital signal.
According to IEEE 802.11a, which is one of the conventional wireless LAN standards, a known signal is inserted in a preamble of the head of a wireless packet. A receiver uses the known signal in a received signal to estimate carrier offset and clock offset to compensate the carrier offset and the clock offset in accordance with the obtained estimated value. However, since the known signal is lost due to noise, estimating and compensating the carrier offset and the clock offset in use of the known signal also poses the problem of a residual offset behind the preamble. To solve such a problem, IEEE 802.11a uses a subcarrier (referred to as pilot subcarrier) of a part of data to transmit a pilot signal, and the receiver uses the received pilot signal to estimate and compensate for the residual offset.
One example of a method for compensating for clock offset by using the pilot signal is disclosed in JP-A 2004-312372 (KOKAI). Angles of phase rotations caused by the clock offset become smaller the closer the subcarriers are to a center frequency, and larger if the subcarriers are more distant from the center frequency. According to FIGS. 15 (A) and (B) in JP-A 2004-312372 (KOKAI), white points represent signal reception points when no clock offset is present, and black points represent phase rotations caused by the clock offset. In contrast, the angles of phase rotations caused by the carrier offset are identical in all pilot subcarriers.
Since the carrier offset and the clock offset occur simultaneously, phase rotations in which influences from the carrier offset and influence from the clock offset are combined occur. Using this occurrence of the phase rotations achieves estimation and compensation of the clock offset by using the received pilot signals in JP-A 2004-312372 (KOKAI).
Meanwhile, a multi input multi output (MIMO) system, using each of a plurality of antennas for a transmitter and a receiver, has received attention in view of its high throughput. Further, a MIMO-OFDM system using both the MIMO and OFDM has been regarded as the most likely next-generation radio communication system. However, transmitting identical pilot signals from a plurality of antennas poses the problem of mutual interference. This interference results in a phenomenon, for instance, in which a strong pilot signal is transmitted in a certain direction, but not in another direction. This phenomenon is called a beam forming influence.
In a draft “Joint Proposal: High throughput extension to the 802.11 Standard: PHY” (Document 1) of IEEE 802.11n, which is a next-generation wireless LAN system in which the MIMO-OFDM system is regarded to be adopted as a standard, devising an idea for transmission patterns of the pilot subcarrier transmitted from a plurality of antennas prevents the influence of beam forming. In pilot subcarrier patterns depicted in Table 17-Pilot values for 40 MHz transmission in Document 1, Nsts is the number of all streams transmitted simultaneously (here, read as the number of all transmission antennas), and ists following the Nsts is the number of streams to be actually transmitted (here, read as the number of transmission antennas). Next to the ists, a −21st, a −7th, a +7th, and a +21st pilot subcarriers of transmit signals are transmitted. For instance, if the number of transmission antennas is four, in a 0th OFDN symbol, a 0th transmission antenna transmits a signal with a pattern of (1, 1, 1, −1), a 1st transmission antenna transmits a signal with a pattern of (1, 1, −1, 1), a 2nd transmission antenna transmits a signal with a pattern of (1, −1, 1, 1), and a 3rd transmission antenna transmits a signal of a pattern (−1, 1, 1, 1).
Here, a pilot subcarrier, for example, a −21st subcarrier is considered. The −21st pilot subcarrier simultaneously transmits signals with patterns of (1, 1, 1, −1) from four transmission antennas. This pattern differs from that of a signal transmitted through another pilot subcarrier. Especially, when the number of transmission antennas is four, a pattern of a signal transmitted from a certain pilot subcarrier is orthogonal to a pattern of a signal transmitted from another pilot subcarrier. Therefore, even if the receiver is present in a null direction of a transmission beam formed of a −7th, a +7th and a +21st pilot subcarrier, a possibility that a transmission beam formed of a −21st pilot subcarrier reaches the receiver becomes high.
In general, a signal transmitted from a transmitter generates reflection diffraction by a feature. If reflection diffraction is generated, the receiver receives the signal transmitted from one transmitter as a plurality of signals via a plurality of paths, so that the envelope of the received signal varies depending on the place and time (referred to as fading). If fading has occurred, even if the transmission beam formed of a pilot subcarrier (in an example given above, −21st pilot subcarrier) is directed in the direction of the receiver, the influence of fading lowers the received power of the −21st, −7th, +7th and +21st pilot subcarriers sometimes. As a result, in the forgoing situation in which the receiver is located in the null direction of the transmission beam formed of the −7th, +7th and +21st pilot subcarriers, there is a possibility that the electric power of the +21st pilot subcarrier received will be lowered. Thereby, the receiving performance of the receiver greatly drops.
As explained above, the combination between the offset compensation technique disclosed in JP-A 2004-312372 (KOKAI) and the pilot subcarrier pattern described in Document 1 cannot receive the pilot signals to compensate for the offset, and deteriorates the reception performance sometimes.